While there is a common belief that the evolution of humans can be traced back to fishlike vertebrate ancestors, pinpointing the origins of bony fish — a key group in this evolution — remains challenging due to the scarcity of primitive fossils.
However, a group of Chinese scientists have shed light on the origin of bony fish by discovering the group's oldest known fossils in South China, highlighting the region as a cradle of early vertebrate evolution.
The fossils of the previously lesser-known bony fish stem group have unveiled their morphology and anatomical features, including jaws, teeth, and braincases.
These findings were featured in two papers published as cover stories in the journal Nature on Thursday.
"Bony fish form the main trunk of the vertebrate tree of life. Their two surviving groups, ray-finned fish and lobe-finned fish, have adapted to a wide variety of environments both in water and on land," said Zhu Min, an academician of the Chinese Academy of Sciences and a professor at the Institute of Vertebrate Paleontology and Paleoanthropology, who led the studies.
Zhu explained that ray-finned fish, with over 30,000 living species, make up most of the fish we know today and thrive in various aquatic environments.
Meanwhile, lobe-finned fish include a group that moved onto land during the Devonian Period (from 416 million years ago to 359 million years ago), eventually leading to all land-dwelling animals with a backbone, including humans.
He pointed out that nearly all early bony fish fossils were already highly specialized ray-finned or lobe-finned fish from the Devonian Period.
Yet, the studies have uncovered fossil evidence from before the divergence during the Silurian Period, which began around 440 million years ago, thus "filling a major gap in the early evolutionary history of 'from fish to human'," he added.
After more than a decade of fieldwork and laboratory research, the team recovered the oldest complete bony fish fossil from the early Silurian deposits of Xiushan, Chongqing, which they named Eosteus chongqingensis. Living around 436 million years ago, the fish measured only 3 centimeters in length.
"The tiny fish, whose fossil is well-preserved from head to tail, displays derived traits resembling early rayfinned fish, such as its streamlined body and single dorsal fin, while also having primitive traits, such as lacking the bony fin rays typical of bony fish," said Zhu Youan, a professor at the Institute of Vertebrate Paleontology and Paleoanthropology.
Zhu Youan said the discovery demonstrates that the set of core bony fish features evolved much earlier than previously thought and refutes the hypothesis that the ancestral bony fish was more similar to lobe-finned fish.
In addition, the team also reconstructed the braincase and tooth structure of the largest known vertebrate of the Silurian Period, known globally as Megamastax amblyodus, from the late Silurian deposits of Qujing, Yunnan province. Living around 423 million years ago, the fish measured over 1 meter in length.
Lu Jing, a professor at the Institute of Vertebrate Paleontology and Paleoanthropology, underscored the significance of technology in the investigation of fossil materials, including the use of tomographic and three-dimensional computer reconstruction techniques.
Lu said that the restoration results suggest a unique dental characteristic of early bony fish — the dentition comprises inner and outer tooth rows, with the inner row consisting of tooth cushions perched on blunt bases.
This structure resolves the longstanding debate over the affinities of isolated tooth cushions from the Silurian Period, which were found in the Baltic region half a century ago, by identifying their correct placement.
"From 3 cm to 1 m, the change in fish sizes suggests a significant rise in oxygen levels during the Silurian Period, and by the late Silurian, when fish size was comparable to those of today, the oxygen levels might have also stabilized," Lu said.
She said the team will continue to utilize the unique fossil evidence from the Silurian Period in South China to uncover the early evolutionary history of "from fish to humans".
